This invention relates to stop-light monitor devices, and more particularly to devices of this type as used in automobiles, in connection with incandescent electric lamps.
In the past a number of different lamp-outage or monitor devices have been proposed and produced for the purpose of indicating malfunctioning of one or several incandescent stop lamps in an automobile. One such device has a flashing-type relay with two coils that buck each other, the flux from one coil being sensitive to the functioning or malfunctioning of the signal lamps. The relay construction, involving bucking coils wound on a magnetic core, represented an appreciable expense and further involved a nicety of adjustment which did not readily lend itself to universal installation. The added cost of the relay was a distinct drawback.
Another prior device also utilized a relay with bucking coils, but the cost was kept down by resorting to encapsulated reed elements. In this device, however, the pilot or indicator lamp did not become energized so as to reveal the normal operation of the stop lamps, but the reverse was true, i.e. the indicator lamp would light up to signal a malfunction. Also, the reed switch was called upon to carry the entire current of the indicator lamp, and these two characteristics constituted drawbacks which reduced the effectiveness and desirability of the device.
Yet another prior device relied upon the surge value which occurred initially, when the incandescent stop lamps were energized. Such reliance on a surge value was considered undesirable, since malfunctioning of the device could occur under certain conditions of use, as for example where a momentary short circuit occured, or where there existed a low resistance leak to ground.
In still another prior device a switch, such as a brake switch for example, was connected in a circuit that caused current to flow through the stop lamps without illuminating them, when the switch was open. This represented an undesirable drain of current from the battery.
Still other prior devices utilized a plurality of silicon-controlled rectifiers and a Zener diode in series with one of two bucking coils, or else an arrangement having multiple switches, in providing the indication of malfunctioning. Here, again, the increased cost was an important factor and an undesirable characteristic, as well as the greater complexity of the circuitry.